1. Field of the Invention
The present invention relates to telecommunications. The invention more particularly relates to wireless telecommunications apparatus, systems and methods which implement data transmission via a plurality of telecommunication channels such as radio channels with variable parameters, including multipath wireless channels. More specifically, the invention relates to pilotless wireless systems with mobile transmitters and/or receivers, although it is not limited thereto.
2. State of the Art
Development of pilotless data transmission and signal processing is an important problem of wireless system design. Pilotless wireless systems provide the highest real data rate by utilizing system capacity exclusively for data transmission and providing all receiving functions without any accompanying pilot signals.
One manner of implementing a pilotless system is using coherent demodulation based on a reference signal and extracting all necessary information from signals-bearing data. In this case, precise estimation and tracking of the carrier phase plays a critical role. A conventional approach to carrier phase estimation is described in John Proakis, “Digital Communications”, McGraw Hill, Fourth Edition, 2000, Sections 6.2.4-6.2.5 which is hereby incorporated herein in its entirety. A new method of carrier phase tracking was proposed in co-owned U.S. Ser. No. 10/628,943 filed Jul. 29, 2003, and entitled “Pilotless, Wireless, Telecommunications Apparatus, Systems and Methods” which is also hereby incorporated by reference herein in its entirety. In that invention, phase tracking is based on reducing and averaging differential quadrature components of received symbols.
Coherent processing, based on carrier phase tracking, provides the maximum possible performance in channels with comparatively slow phase changing and comparatively high signal-to-noise ratio (SNR). If one of those conditions is not satisfied, the coherent demodulator loses its advantages. For example, when the bit error rate BER>0.01, phase estimation becomes less precise. The SNR penalty depends on the method of phase tracking but it may reach 1-2 dB. However, much more performance loss can be caused by fast phase changing or, especially, phase jumping. Fast phase changing and phase jumping are typical phenomenon in communications involving mobile clients because in the mobile environment the multipath configuration may change instantly. This change can cause coherent detection degradation even at a comparatively high SNR because any phase tracking algorithm, based on symbol averaging, is not capable of instantly estimating the phase changing. As a result, the coherent receiver provides a long sequence (burst) of errors after phase jumping even at a comparatively high SNR. This is the reason why wireless system designers consider approaches other than coherent processing for pilotless systems.
For multipath radio channels in a mobile environment, a promising manner of implementing a pilotless system involves the utilization of noncoherent signal processing which does not need any information about the initial phase of the reference signal and consequently does not require phase tracking at all. In the case of phase modulation, noncoherent processing can be only used in combination with Differential Phase Shift Keying (DPSK) in contrast to coherent processing which can be used with both DPSK and PSK. As is emphasized in Dariush Divsalar, and Marvin Simon, “Multiple-Symbol Differential Detection of MPSK”, IEEE Transactions on Communications, vol. 38, N3, March 1990 (which is hereby incorporated by reference in its entirety), for multipath channels with fast phase changing, a DPSK-with-noncoherent-processing scheme is the only way to provide robust data transmission.
Conventional noncoherent DPSK processing includes a two-symbol interval for making a decision. With a two-symbol interval, if a phase jump occurs during the symbol interval, only one error will take place. However, two-symbol non-coherent DPSK suffers from a performance penalty when compared to ideal coherent DPSK. For example, at BER=10−5 the penalty is about 0.75 dB for DBPSK (Differential Binary PSK), and about 2.2 dB for DQPSK (Differential Quadrature PSK). For M-ary DPSK (DMPSK) the penalty increases with increasing M.
A well-known way to mitigate the performance loss of the non-coherent receiver and preserving its advantages is to increase the interval of the non-coherent processing. This approach, known as multisymbol non-coherent detection (or processing) was considered in the previously incorporated article by Dariush Divsalar, and Marvin Simon. The conventional approach to multisymbol processing is based on allowing the observation interval over which symbol decisions are made to be more than a two-symbol interval while at the same time making a joint decision on several symbols simultaneously as opposed to symbol-by-symbol decisions. An obvious disadvantage of the joint decision on several symbols is the additional symbol delay which results in the receiver. In addition, the joint decision procedure is undesirable for some decoding algorithms.